1. Field of the Invention
This invention relates to a control for changing the gear ratios of an automatic transmission. More particularly, it pertains to a control of the hydraulic pressure supplied to the clutches and brakes of an automatic transmission whose engagement and disengagement selectively produce the various speed ratios of the transmission.
2. Description of the Prior Art
The speed ratio of an automatic transmission can be upshifted by disengaging a first clutch, whose engagement holds a member of the gear set against rotation and causes the lower speed ratio to be produced, and by engaging a second clutch or brake, whose engagement combined with the disengagement of the first clutch holds another member of the gear set and causes operation at the higher speed ratio. Ideally, the output torque of the transmission is constant or changes smoothly and imperceptibly during the ratio change. Hydraulic pressure is supplied to certain clutches and brakes, and others of these are vented selectively to produce engagement and disengagement.
A power-on upshift of an automatic transmission involves an output torque transient whose earliest portion produces decreasing acceleration of the motor vehicle and whose latter portion produces increasing acceleration as the transmission speed ratio increases. The upshift transient has a first or torque phase, during which torque changes occur on the clutches and transmission output without a speed change and thus without inertia torques. The transient has also a second or inertial phase, during which the clutch elements are accelerated to their new speeds with associated inertia torque. During the torque phase, the output torque of the transmission is a function of the torque produced by the engine and the torque carried by the clutches. During the inertia phase, the output torque of the transmission is a function only of clutch torque, in a transmission where an overrunning clutch provides the reaction at the higher gear ratio. Where a friction element such as a clutch or brake is used instead of an overrunning clutch for this purpose during the inertia phase, the output torque of the transmission is a function of the engine torque, clutch torque and the torque on the reaction friction element.
During this transient, a passenger first senses decreasing acceleration during the torque phase followed by an abrupt change to an increasing acceleration as the torque is transferred from one friction element to another. If the clutch torque is not carefully limited, the change in output torque when the speed change is completed will be sensed by the passenger as a decreasing acceleration. The prior art has taught that the decreasing output torque during the torque phase is an inherent characteristic of a power-on upshift and is unavoidable, particularly so, when the output torque before the shift equals the output torque after the shift.